In conjunction with the recent rapid advances of portable electronic equipment and communications instruments, nonaqueous electrolyte secondary batteries having a high energy density are strongly demanded from the aspects of cost, size and weight reductions. Approaches known in the art to increase the capacity of such nonaqueous electrolyte secondary batteries include, for example, use as negative electrode material of oxides of B, Ti, V, Mn, Co, Fe, Ni, Cr, Nb, and Mo and composite oxides thereof (Patent Documents 1 and 2); application as negative electrode material of M100−xSix wherein x≧50 at % and M=Ni, Fe, Co or Mn which is obtained by quenching from the melt (Patent Document 3); use as negative electrode material of silicon oxide (Patent Document 4); and use as negative electrode material of Si2N2O, Ge2N2O or Sn2N2O (Patent Document 5).
Among others, silicon oxide is represented by SiOx wherein x is slightly greater than the theoretical value of 1 due to oxide coating, and is found on X-ray diffractometry analysis to have the structure that amorphous silicon with a size from several nanometers to several tens of nanometers is finely dispersed in silica. Silicon oxide offers a greater battery capacity than the currently available carbon by a factor of 5 or 6 on a weight basis and relatively good cycle performance due to a less volume expansion. For these reasons, batteries using silicon oxide as the negative electrode material are regarded fully effective for use in portable electronic equipment such as mobile phones, lap-top computers and tablets. When the automotive application is considered, however, these batteries are insufficient in cycle performance and expensive.